The road less traveled: activating an oncogenic kinase
Authors: Madhuri Paul, David A. Fruman
Journal: Trends in Pharmacological Sciences (2023)

Elevated phosphoinositide 3-kinase (PI3K) activity in human tumors has prompted widespread efforts to develop chemical PI3K inhibitors for oncology indications. In an innovative new study, Gong et al. report the discovery of a highly selective activator of the PI3Kα isoform, with promising activity in assays of nerve regrowth and cardioprotection from ischemia–reperfusion injury (IRI). Elevated phosphoinositide 3-kinase (PI3K) activity in human tumors has prompted widespread efforts to develop chemical PI3K inhibitors for oncology indications. In an innovative new study, Gong et al. report the discovery of a highly selective activator of the PI3Kα isoform, with promising activity in assays of nerve regrowth and cardioprotection from ischemia–reperfusion injury (IRI). Many of the most prevalent human oncogenes encode activated forms of protein or lipid kinases. This knowledge has spurred kinase inhibitor development programs that have yielded many new drugs that are now standard of care in precision oncology. Kinase inhibitors are also valuable tools for basic research. However, throughout decades of kinase inhibitor development, researchers have largely ignored the potential applications of kinase activator compounds in research and clinical medicine. A recent study in Nature provides compelling evidence that kinase activator development is both feasible and worthwhile [1.Gong G.Q. et al.A small-molecule PI3Kα activator for cardioprotection and neuroregeneration.Nature. 2023; 618: 159-168Crossref PubMed Scopus (1) Google Scholar]. Gong et al. set out to identify small molecules that increase activity of PI3Kα, a member of the class I PI3K family of lipid kinases [1.Gong G.Q. et al.A small-molecule PI3Kα activator for cardioprotection and neuroregeneration.Nature. 2023; 618: 159-168Crossref PubMed Scopus (1) Google Scholar]. PI3K enzymes are activated by diverse cellular receptors to produce phosphoinositide products that recruit cytoplasmic proteins to membrane-associated signaling complexes [2.Fruman D.A. et al.The PI3K pathway in human disease.Cell. 2017; 170: 605-635Abstract Full Text Full Text PDF PubMed Scopus (1355) Google Scholar]. The PI3Kα isoform is a central mediator of cell proliferation and survival driven by activated growth factor receptors and RAS GTPases. Furthermore, activating mutations in PIK3CA encoding PI3Kα are prevalent in human cancers. These properties validated PI3Kα as an attractive target for drug development. Several companies have developed PI3Kα-selective inhibitors, one of which (alpelisib; BYL719) is approved in combination with fulvestrant for treatment of hormone receptor-positive breast cancer with PIK3CA mutations (Figure 1) [3.André F. et al.Alpelisib for PIK3CA-mutated, hormone receptor-positive advanced breast cancer.N. Engl. J. Med. 2019; 380: 1929-1940Crossref PubMed Scopus (1220) Google Scholar]. PI3Kα-selective inhibitors might also be useful in tissue overgrowth syndromes associated with mosaic PIK3CA mutations [4.Madsen R.R. et al.Cancer-associated PIK3CA mutations in overgrowth disorders.Trends Mol. Med. 2018; 24: 856-870Abstract Full Text Full Text PDF PubMed Scopus (141) Google Scholar]. Other PI3K isoforms are also promising targets: inhibitors of the PI3Kδ isoform are used in second- and third-line treatment of hematological malignancies, and agents targeting PI3Kβ or PI3Kγ remain under investigation. Overall, thousands of small molecules have been disclosed that inhibit one or more PI3K isoforms with low to mid-nanomolar potency in cells and have drug-like properties. Concurrently, evidence has emerged that increasing PI3Kα activity might have therapeutic benefit in certain pathophysiological conditions, including nerve damage and IRI following myocardial infarction [5.Rossello X. et al.The role of PI3Kα isoform in cardioprotection.Basic Res. Cardiol. 2017; 112: 66Crossref PubMed Scopus (50) Google Scholar]. These findings provided the impetus for developing PI3Kα activator compounds. To pursue this ambitious goal, experts in PI3K biology and drug development at University College London partnered with AstraZeneca via its Open Innovation program [1.Gong G.Q. et al.A small-molecule PI3Kα activator for cardioprotection and neuroregeneration.Nature. 2023; 618: 159-168Crossref PubMed Scopus (1) Google Scholar]. This collaboration enabled high-throughput screening to identify compounds that increase PI3Kα activity in an in vitro assay, and in cells using increased AKT phosphorylation as a readout. Lead optimization resulted in the molecule UCL-TRO-1938, referred to as 1938. Kinetic and structural studies demonstrated that the molecule is an allosteric activator that binds the kinase domain distal to the ATP-binding site and does not overlap with the binding surface for the inhibitor BYL719 (Figure 1). A crystal structure of 1938 bound to a PI3Kα fragment indicated that the compound changes the PI3Kα conformation in a manner distinct from one of its endogenous activators (phosphotyrosine peptides) and from oncogenic point mutations. In accord, 1938 increases the activity of wild-type PI3Kα as well as various mutant forms, including the hotspot mutants E545K and H1047R. 1938 was highly selective for PI3Kα versus other PI3K isoforms, which could be explained by structural modeling of the binding site. With this selective activator in hand, the authors proceeded to test its activity in cellular and animal models. As one would expect from a PI3Kα activator, addition of 1938 to cells increased production of PI3K lipid products as well as phosphorylation of AKT and many other proteins. Notably, 1938 alone was sufficient to induce cell proliferation in wild-type fibroblasts but not in PI3Kα knockouts, illustrating both the biological potency and selectivity of this molecule. The most exciting results emerged from studies of tissue protection in rodents and explanted tissues. Previous work suggested that PI3Kα is a central regulator of IRI; mouse hearts expressing constitutively active PI3Kα were protected, whereas IRI was enhanced by treatment with selective PI3Kα inhibitors [5.Rossello X. et al.The role of PI3Kα isoform in cardioprotection.Basic Res. Cardiol. 2017; 112: 66Crossref PubMed Scopus (50) Google Scholar,6.Yellon D.M. et al.The RISK pathway leading to mitochondria and cardioprotection: how everything started.Basic Res. Cardiol. 2023; 118: 22Crossref PubMed Scopus (0) Google Scholar]. In rat ex vivo and mouse in vivo models of IRI, 1938 reduced the cardiac infarct size by ~50% and increased AKT phosphorylation. Chemical agents that regenerate damaged nerves would have enormous potential to improve the recovery of patients from peripheral nerve injury with possible application in more severe central nervous system damage. Gong et al. found that neurite outgrowth in dissociated rat dorsal root ganglion cultures was augmented in the presence of compound 1938 but suppressed by BYL719 [1.Gong G.Q. et al.A small-molecule PI3Kα activator for cardioprotection and neuroregeneration.Nature. 2023; 618: 159-168Crossref PubMed Scopus (1) Google Scholar]. These results prompted the authors to test 1938 in vivo using a rat sciatic nerve crush model. Remarkably, treatment with the PI3Kα activator significantly improved several measures of nerve regeneration and muscle innervation. Together, these results provide a proof of concept that chemical activation of PI3Kα is a feasible approach for cardioprotection and neuroregeneration (Figure 1) that warrants further investigation. If these strategies reach the clinic, PI3Kα activators might have even broader application than those inhibitors that have been approved for use in patients with a subset of breast cancers. An obvious concern with the use of PI3Kα activators in patients is whether such treatment would have tumor-promoting potential. The authors point out several factors that mitigate this concern. First, activator effects are transient and likely reversible after treatment withdrawal. Second, PIK3CA mutations generally have weak oncogenic potential on their own. Third, patients with PIK3CA-driven mosaic overgrowth syndromes are not predisposed to malignant tumors. Nevertheless, further testing is needed to determine whether PI3Kα activators promote tissue overgrowth, vascular malformations, or tumorigenesis in the context of other oncogenes. In addition, the duration of treatment necessary to achieve and sustain nerve regeneration is not yet clear; in the sciatic nerve model developed by Gong et al., a single local dose was accompanied by sustained delivery via an implanted minipump for the 3 weeks of the experiment [1.Gong G.Q. et al.A small-molecule PI3Kα activator for cardioprotection and neuroregeneration.Nature. 2023; 618: 159-168Crossref PubMed Scopus (1) Google Scholar]. One underappreciated aspect of this study is the potential of compounds such as 1938 as research tools. To fully understand the spectrum of kinase functions, it is essential to use both loss- and gain-of-function approaches. The field now has many chemical inhibitors for loss-of-function studies of PI3K that have been invaluable for understanding the shared and unique functions of specific isoforms in different physiological systems. By contrast, gain-of-function strategies have relied on genetic tools, such as constitutively active enzymes, which may not be amenable to the fine tuning that is possible by titrating an activator compound. Gong et al. reported that excessive activation of PI3Kα by 1938 triggered broader signaling responses compared with insulin [1.Gong G.Q. et al.A small-molecule PI3Kα activator for cardioprotection and neuroregeneration.Nature. 2023; 618: 159-168Crossref PubMed Scopus (1) Google Scholar], whose receptor engages only a fraction of available PI3Kα, yet 1938 generates fewer PI3K phosphoinositide products compared with PDGF, whose receptor engages both PI3Kα and PI3Kβ. Could a PI3Kα activator have applications in oncology? Growing evidence supports the idea that cancer cells are susceptible to ‘oncogene overdose’, which overloads the stress response capacity to cause cell death [7.Chen Z. et al.Signalling thresholds and negative B-cell selection in acute lymphoblastic leukaemia.Nature. 2015; 521: 357-361Crossref PubMed Scopus (102) Google Scholar, 8.Rajan S.S. et al.The mechanism of cancer drug addiction in ALK-positive T-Cell lymphoma.Oncogene. 2020; 39: 2103-2117Crossref PubMed Scopus (6) Google Scholar, 9.Gutierrez-Prat N. et al.DUSP4 protects BRAF- and NRAS-mutant melanoma from oncogene overdose through modulation of MITF.Life Sci. Alliance. 2022; 5e202101235Crossref PubMed Scopus (2) Google Scholar]. In this context, intermittent dosing with oncogenic kinase inhibitors has a twofold benefit through cycles of oncogene inhibition and overactivation [10.Amin A.D. et al.Oncogene overdose: too much of a bad thing for oncogene-addicted cancer cells.Biomark. Cancer. 2015; 7: 25-32PubMed Google Scholar]. The oncogene overdose effect might be augmented by a kinase activator, especially with compounds such as 1938, which act on both the mutated and endogenous isozymes. It is intriguing to consider that alternating cycles of inhibitor and activator could achieve optimal tumor regression. After decades of traveling on crowded highways of kinase inhibitor development, the example set by compound 1938 should motivate drug developers to blaze new trails to generate kinase activator compounds for research and medicine. None declared by authors.